1. Field of the Invention
The present invention relates generally to a color cathode ray tube apparatus, and more particularly to a color cathode ray tube apparatus wherein three in-line electron beams consisting of a center beam and a pair of side beams, which all travel in the same plane, are focused by a common magnetic field focusing type electron lens employed commonly for the three electron beams.
2. Description of the Related Art
There is a demand for a color cathode ray tube (CRT) apparatus capable of displaying a clear image with a higher resolution. In order to achieve such a CRT apparatus, it is necessary to reduce the beam spot diameter on a phosphor screen of an electron beam emitted from an electron gun assembly.
In order to meet the demand, Published Examined Japanese Patent Application (PEJPA) No. 57-6662 proposes a color CRT apparatus wherein two side cathode in a group of three cathodes are inclined with respect to the tube axis so as to make three in-line electron beams intersect in a magnetic field focusing type electron lens and to equalize the intensity of the magnetic field focusing type electron lens acting on the electron beams, and the occurrence of astigmatism or coma-aberration in the pair of side beams is prevented, whereby the three electron beams are focused in an equal focused state.
In the apparatus having the above structure, the pair of side beams are made to intersect in the lens and then travel in a direction away from the center beam. The side beams are intensely deflected towards the center beam by an electrostatic deflecting plate, and the three electron beams are converged at one point on a center region of the screen. Although the axis of arrangement of the three electron beams is rotated about the tube axis, such a rotation is corrected by arranging the three cathodes with an inclination to the horizontal axis of the deflected magnetic field (the horizontal axis perpendicular to the tube axis). Thereby, a convergence error of the three electron beams is prevented.
However, in the apparatus of the above structure, the two side cathodes of the three cathodes must be arranged at a precise inclination with respect to the center cathode. It is difficult to assemble the electron gun assembly with high precision. In addition, after the three electron beams have passed through the common magnetic field focusing type electron lens, the pair of side beams which travel away from the center beam are intensely deflected towards the center beam by the electrostatic deflecting plate. Thus, a deflection aberration given to the three electron beams is high, and the three electron beams cannot be converged on the phosphor screen in the same focused state. Therefore, practically required image quality cannot be obtained.
As has been described above, if the magnetic field focusing type electron lens is used, the three electron beams cannot be converged at one point on the phosphor screen and the image plane is rotated with respect to the object point. Accordingly, if the horizontal in-line three electron beams are focused by the magnetic field focusing type electron lens, a considerable convergence error occurs at the peripheral region on the screen. This convergence error cannot be completely corrected even if the distributions of horizontal and vertical deflection magnetic fields of the deflector are adjusted.
Furthermore, in the conventional apparatus, the deflection aberration is large, and the three electron beams cannot be converged on the phosphor screen in the same focused state.